It avoids being fashionable and therefore never appears antiquated. Unlike fashionable design, it lasts many years — even in today’s throwaway society.
Long-lasting is a tricky word but still Rams’ statement is very clear on its meaning. It doesn’t mean an object needs to just last a long time itself, it means timeless.
Of-course that “good design is long-lasting” means to last long time, but not just the integrity of an object but its purpose, its appealing, its beauty.
Timeless, on the other hand, is something that is not affected by the passage of time or changes in fashion. It’s independent of time. It’s not ephemeral.